itunes deleted my library from my ipod. now i only have new purchases. how can I get my whole library back onto my ipod?

I just transferred itunes to a new computer. When I plugged in my ipod, I was asked if I wanted to erase my old library and put the new on on. I said yes because my whole library is on this computer. Now that it's done with the sync, I only have the three albums that I just purchased and all of the rest is gone. How can I sync my whole music library back onto this device?